The cord of a conventional Trim phone to connect with its handset and base needs three cores, i.e., the on-hook terminal, the positive output terminal of bridge rectifier and the ground terminal. The operation of the conventional calling line identification circuit is based on the differential input configuration. It is well known that the differential input configuration has two input terminals, so that the cord of a conventional Trim phone must have more than four cores to connect with the conventional calling line identification circuit for having a caller identification function. But, in view of the fact that the cord of a trim phone to connect with its handset and base only has four cores, the trim phone is incapable of connecting with the conventional calling line identification circuit for identifying a calling line.
FIG. 2 illustrates the conventional three-wire cord telephone of the type with a handset H, base B, and cord C (also shown in FIG. 2). In FIG. 3 the dashed line schematically indicates the cord C and the wires are labeled 1, 2, and 3 (ground). Wires 1 and 2 branch from one terminal of a bridge rectifier circuit 10 and wire 1 includes a hook switch 30. The conventional phone of FIG. 3 includes no caller identification unit.
FIG. 4 illustrates the conventional circuitry for caller identification. The identification unit 70 is coupled directly to the tip and ring lines from the telephone company through an optional surge protector.
Because of this conventional circuitry, if a caller identification device is to placed into the handset H then the cord C must be a five-wire cord. However, standard telephone cords and jacks only contain four wires.